doc: clarify the description of du --separate-dirs

* src/du.c (usage): Clarify that --separate-dirs doesn't exclude
all directories.
* doc/coreutils.texi (du invocation): Avoid implying that -S
excludes the size of any non directory entries for a directory.
Also don't mention st_size as it's dependent on --apparent-size.
Reported by C de-Avillez in <https://launchpad.net/bugs/1187044>
maint: update gitignore entries

* .gitignore: Exclude the generated doc/manual directory,
and also the src/fs-latest-magic.h file referenced in README-release.
tests: use appropriate precision when printing float limits

* src/getlimits.c (print_float): Adjust to use the ftoastr module,
which uses the appropriate precision so that no info is lost.
* cfg.mk (sc_prohibit_continued_string_alpha_in_column_1): Exclude od.c
fixes http://bugs.gnu.org/14650